1. Field of the Invention
The present invention relates to electrical connectors generally, and more particularly to an electrical connector having a controlled impedance and a high density of signal contacts by using a groundplane in proximity to the signal contracts.
2. Description of the Prior Art
Conventional types of connectors have been used heretofore for connection of circuits on motherboards and daughterboards, in computer equipment or in similar applications, and they have generally been reliable in operation. However, there have been problems and in the last few years they have been increasing in magnitude, especially when contact spacings are reduced, to reduce the sizes of connectors and/or to increase the number of contacts, or when the interconnected circuits are designed to use advances in technology which make it possible to transmit large volumes of data at high speeds. Such problems have included loss of transmitted signals, interference between signals or "cross-talk" and interference from extraneous signals. The existence of such increasing problems have been generally recognized, but satisfactory solutions have not been apparent.
Some of these problems have been attributed to poor ground connections. For example, ground connectors tend to develop electrostatic charges when high volumes of signals are transmitted at high speeds. A shift in voltage between groundplanes of two interconnected circuits may result in loss of reference levels in electronic circuitry. Mismatched impedances between circuitry and connectors causes reflections and the production of undesirable standing wave phenomena, with corresponding errors in transmitting data, in the case of transmitting data signals. It has also been recognized that cross-talk between signal paths increases with frequency and with decreases in spacing between signal contacts. This problem is affected to a substantial extent by the characteristics of the ground connection which is common to the signal paths.
Typically, one or more connector pins have been used in the past for ground connections and, in some cases, each pin used for signal transmission may have an associated adjacent pin used for a ground connection, in an attempt to minimize cross-talk problems. It has been found that this does not provide an adequate solution because there may nevertheless be substantial impedances in the ground connections and also, this solution requires many more connector pins. Moreover, if the number of ground pins were increased so as to use two or more pins for each signal pin, it would impose severe space limitations, increase insertion forces, and provide a less continuous shielding field than a groundplane.
Another problem with prior constructions relates to the impedance characteristics of the signal paths. Each signal path of an electrical connector, with conductor length greater than 0.05 times wavelength, may be considered as an electrical transmission line having a certain characteristic impedance determined by its resistance, inductance, and distributed capacitance per unit length. At relatively low signal transmission velocities with associated lower frequency and longer wavelength, the actual impedance of the path is not usually important. However, at high velocities, the path may produce reflections, resonances and standing wave phenomena when there is a substantial mismatch between the characteristic impedances of the circuits connected thereto. It has also been observed that it is especially desirable that the characteristic impedances of all paths be substantially the same within a given circuit path, and targeted to the characteristic impedance of the logic type used, so as to facilitate design of the connected circuits.
Such impedance characteristics of an electrical connector may also affect different types of circuits in different ways. For example, some systems use mixed logic such as emitter coupled logic (ECL), transistor to transistor logic (TTL) and/or complimentary metal oxide semiconductor (CMOS) logic. Each of these logic circuits perform best at different target system characteristic impedances. Thus, it would be beneficial to provide an electrical connector capable of closely controlling characteristic impedances to match the different logic sections of a printed circuit board. To date, no such connectors are available which meet this entire list of needs.